Communication network control system, control method thereof, and non-transitory computer readable medium of control program

ABSTRACT

A communication network control system ( 1 ) eliminates, in a communication network (G) in which a plurality of nodes (N i ) are connected via a plurality of links ( 2 ), a node (N x ) having a trouble and implements a reconnection in the communication network (G). The nodes (N i ) each comprise: a path information calculation means ( 3 ) that calculates path information of the communication network (G) when a trouble occurs in an adjacent node (N x ); an order information calculation means ( 4 ) that calculates, for a set of reconnection destination candidate nodes (N i ), order information in which the reconnection destination candidate nodes are sequenced in order of inter-node distance on the path information calculated by the path information calculation means ( 3 ); and a determination information calculation means ( 5 ) that calculates, on the basis of the order information calculated by the order information calculation means ( 4 ), such a combination that the inter-node distance on the path information in the whole communication network (G) after the reconnection becomes larger and that determines the calculated combination as a reconnection destination node of the local node (N i ).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage application of InternationalApplication No. PCT/JP2013/004853 entitled “COMMUNICATION NETWORKCONTROL SYSTEM, CONTROL METHOD THEREOF, AND NON-TRANSITORY COMPUTERREADABLE MEDIUM OF CONTROL PROGRAM,” filed on Aug. 13, 2013, whichclaims the benefit of the priority of Japanese Patent Application No.2013-001308, filed on Jan. 8, 2013, the disclosures of each of which arehereby incorporated by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a communication network control system,a control method and a control program thereof for controlling acommunication network with a plurality of nodes mutually connected vialinks.

BACKGROUND ART

In the field of network topology, “merit as a graph when applied to acommunication network,” has been one of the research subjects in recentyears, where, for example, the advantage of Ramanujan graphs has beendemonstrated.

CITATION LIST Patent Literature

PTL 1: Japanese Patent Application Laid-Open No. 2007-014030

PTL 2: Japanese Patent Application Laid-Open No. H09-284324

SUMMARY OF INVENTION Technical Problem

For example, Ramanujan graphs have been applied to communicationnetworks and power networks as physical/logical topologies. On the otherhand, there has arisen a problem of whether it is possible or not tomaintain the same degree of efficiency and robustness as those of anoriginal graph even when a node or a link is eliminated from theoriginal graph owing to a failure (breakdown or the like) in the node orlink.

In this respect, already known is a communication network control systemwhich reconfigures the communication network when a failure occurs in anode or the like (for example, PTL 1 and 2 are referred to). Further, asa method in which the problem is partially solved with respect to aRamanujan graph mentioned above, the ZigZag method is known, forexample. The ZigZag method is a method of constructing, from an originalRamanujan graph, a Ramanujan graph with a larger size than that of theoriginal one, where it is possible to repeatedly construct a Ramanujangraph with a size being m times that of the original one (m: integerequal to or larger than 3).

The present invention has been conceived in order to solve theabove-described problem, and accordingly, the main objective is toprovide a communication network control system, a control method and acontrol program thereof which can optimally reconstruct thecommunication network while maintaining the characteristics of theoriginal graph even when a failure occurs in a node or a link includedin the communication network.

Solution to Problem

One aspect of the present invention for achieving the above-describedobjective is a communication network control system for, in thecommunication network with a plurality of nodes mutually connected via aplurality of links, eliminating any of the nodes having a failure andperforming reconnection within the communication network, thecommunication network control system being characterized by that atleast one of the plurality of nodes comprises: a route informationcalculation means for calculating route information in the communicationnetwork when a failure occurs in an adjacent node; a ranking informationcalculation means for calculating, with respect to a set of destinationcandidate nodes to which the reconnection may be made, rankinginformation in which the destination candidate nodes are ranked in orderof their distances in the route information calculated by the routeinformation calculation means; and a determination informationcalculation means for calculating, on the basis of the rankinginformation calculated by the ranking information calculation means, acombination which causes node-to-node distances in the route informationto be large for the set of destination candidate nodes of thereconnection, over the entire communication network after thereconnection, and determining the calculated combination as areconnection destination node of the own node.

Another aspect of the present invention for achieving theabove-described objective may be a control method of a communicationnetwork control system for, in the communication network with aplurality of nodes mutually connected via a plurality of links,eliminating any of the nodes having a failure and performingreconnection within the communication network, wherein the controlmethod is characterized by that it comprises: a step of calculatingroute information in the communication network when a failure occurs inan adjacent node; a step of calculating, with respect to destinationcandidate nodes to which the reconnection may be made, rankinginformation in which the destination candidate nodes are ranked in orderof their distances in the calculated route information; and a step ofcalculating, on the basis of the calculated ranking information, acombination which causes node-to-node distances in the route informationto be large over the entire communication network after thereconnection, and determining the calculated combination as areconnection destination node of the own node.

Still another aspect of the present invention for achieving theabove-described objective may be a control program of a communicationnetwork control system for, in the communication network with aplurality of nodes mutually connected via a plurality of links,eliminating any of the nodes having a failure and performingreconnection within the communication network, wherein the controlprogram is characterized by that it causes a computer to execute: aprocess of calculating route information in the communication networkwhen a failure occurs in an adjacent node; a process of calculating,with respect to destination candidate nodes to which the reconnectionmay be made, ranking information in which the destination candidatenodes are ranked in order of their distances in the calculated routeinformation; and a process of calculating, on the basis of thecalculated ranking information, a combination which causes node-to-nodedistances in the route information to be large over the entirecommunication network after the reconnection, and determining thecalculated combination as a reconnection destination node of the ownnode.

Advantageous Effects of Invention

According to the present invention, it is possible to provide acommunication network control system, a control method and a controlprogram thereof which can optimally reconstruct the communicationnetwork while maintaining the characteristics of the original graph evenwhen a failure occurs in a node or a link included in the communicationnetwork.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a functional block diagram of a communication network controlsystem according to an exemplary embodiment of the present invention.

FIG. 2 is a diagram showing an example of a communication networkaccording to an exemplary embodiment 1 of the present invention.

FIG. 3 is a block diagram showing a schematic system configuration ofeach node in a communication network control system according to theexemplary embodiment 1 of the present invention.

FIG. 4 is a diagram showing an example of ranking information anddistance information in shortest route information.

FIG. 5 is a flow chart showing a flow of a communication network controlmethod according to the exemplary embodiment 1 of the present invention.

FIG. 6 is a diagram showing an example of a communication networkaccording to an exemplary embodiment 2 of the present invention.

FIG. 7 is a block diagram showing a schematic system configuration of arepresentative node in a communication network control system accordingto the exemplary embodiment 2 of the present invention.

FIG. 8 is a diagram showing an example of ranking information anddistance information in shortest route information.

FIG. 9 is a flow chart showing a flow of a communication network controlmethod according to the exemplary embodiment 2 of the present invention.

DESCRIPTION OF EMBODIMENTS

Hereinafter, exemplary embodiments of the present invention will bedescribed with reference to drawings. FIG. 1 is a functional blockdiagram of a communication network control system according to anexemplary embodiment of the present invention.

The communication network control system 1 of the present exemplaryembodiment for a communication network G with a plurality of nodes N_(i)(i=0 to n−1) mutually connected via a plurality of links 2, eliminatesany of the nodes N_(i), having a failure and performs reconnectionwithin the communication network G. At least one node N_(i), among theplurality of nodes N_(i) in the communication network control system 1comprises a route information calculation means 3 which calculates routeinformation in the communication network G, a ranking informationcalculation means 4 which calculates ranking information with respect todestination candidate nodes N_(i) to which reconnection may be made, anda determination information calculation means 5 which determines areconnection destination node N_(i) of the own node N_(i). The routeinformation calculation means 3 calculates route information in thecommunication network G when a failure occurs in an adjacent node N_(i).The ranking information calculation means 4 calculates, with respect toa set consisting of candidate nodes for a reconnection destination,ranking information in which the candidate nodes are ranked in order oftheir distances in the route information calculated by the routeinformation calculation means 3. On the basis of the ranking informationcalculated by the ranking information calculation means 4, thedetermination information calculation means 5 calculates a combinationwhich causes node-to-node distances in the route information to be largeover the entire communication network G after reconnection, and on thebasis of the calculated combination, determines a candidate node N_(j)(here, j≠i and j=0 to n−1) for a reconnection destination of the ownnode N_(i). By that way, it is possible to optimally reconstruct thecommunication network G while maintaining the characteristics of theoriginal graph even when a failure occurs in a node or a link in thecommunication network G.

Exemplary Embodiment 1

FIG. 2 is a diagram showing an example of a communication networkaccording to an exemplary embodiment 1 of the present invention. Asshown in FIG. 2, in the communication network G according to the presentexemplary embodiment 1, a plurality of nodes N_(i) are mutuallyconnected via links 2. Here, for example, the nodes N_(i) are routers,and the links 2 are each an optical fiber or a coaxial cable, which aremedia for communication. Besides, for example, the communication networkG is constituted by n-number of nodes N_(i) (i=0 to n−1).

Here, in the present exemplary embodiment 1, for example, a Ramanujangraph is adopted as a physical/logical topology of the communicationnetwork G. The Ramanujan graph belongs to graphs of the class calledExpander, as a graph satisfying the following demands: (1) time forcommunication between any two nodes N_(i) and N_(j) is short, (2) aplurality of routes exist between any two nodes N_(i) and N_(j) (3) thenumber of links among nodes N_(i) increases with increase in thecommunication network size (=the number of nodes) in only a linear orsmaller order relationship. Although the nodal degree of the Ramanujangraph is, for example, a natural number multiple of 4 in the presentexemplary embodiment 1, it does not need to be limited to that value,but may be any value.

Further, a definition of the Ramanujan graph will be further describedin detail below. Assuming the communication network G to be a k-regulargraph with n-number of nodes, and then lining up eigenvalues of theadjacency matrix A of the communication network G in descending order,the following expression (1-1) stands (refer to, for example, AlexanderLubotzky, R. Phillips and Peter Sarnak, “Ramanujan graphs” Combinatorica8, 261-277, 1988).[math 1]k=λ ₁(A(G))≧λ₂(A(G))≧ . . . ≧λ_(n-1)(A(G))≧−k  equation (1-1)

Among the non-trivial eigenvalues other than λ₁, one having the largestabsolute value is expressed as λ(G).

$\begin{matrix}\left\lbrack {{math}\mspace{14mu} 2} \right\rbrack & \; \\{{\lambda\left( {A(G)} \right)} = {\max\limits_{{\lambda_{i}} < k}{\lambda_{i}}}} & {{equation}\mspace{14mu}\left( {1\text{-}2} \right)}\end{matrix}$

In that case, a graph having an eigenvalue satisfying an inequality(1-3) is referred to as a Ramanujan graph.[math 3]λ(A(G))≦2√{square root over (k−1)}  inequality (1-3)

Next, characteristics of the Ramanujan graph will be described.

A first characteristic is its being a k-regular graph. That is, thedegree of every node N_(i) is k. A second characteristic is that itsexpansion constant is large. The expansion constant is defined by afollowing expression (1-4). Here, λ₂ (L(G)) is the second smallesteigenvalue of the Laplacian matrix of the graph G.

$\begin{matrix}\left\lbrack {{math}\mspace{14mu} 4} \right\rbrack & \; \\{{c^{*} \geq {\frac{1}{2}\left( {1 - \frac{\lambda_{n - 1}\left( {A(G)} \right)}{k}} \right)}} = \frac{\lambda_{2}\left( {L(G)} \right)}{2\; k}} & {{inequality}\mspace{14mu}\left( {1\text{-}4} \right)}\end{matrix}$

A third characteristic is that its internal circumference is long. Here,the internal circumference is referred to as the length of the shortestone of loops not passing the same link twice in the graph (closedroutes) (refer to, for example, Toyokazu Hiramatsu and Hiroshi Chinen,“Introduction to finite mathematics—finite upper half plane andRamanujan graph”, Makino Shoten, 2003, ISBN4-434-03407-3). A fourthcharacteristic is that its diameter is short. Here, the diameter isreferred to as the longest one of shortest distances each being thatbetween any two points in the graph.

According to the communication network control system 1 in the presentexemplary embodiment 1, it becomes possible, when one of the nodes N_(i)has been eliminated, to maintain the Ramanujan graph described above ora graph based on the Ramanujan graph by only reconnecting a link 2having been connected to the eliminated node N_(i) to another nodeN_(i). That is, it is possible to maintain the advantageouscharacteristics of the Ramanujan graph, which are a short communicationtime between any two nodes N_(i), existence of a plurality of routesbetween any two nodes N_(i), and increase of the number of links betweenthe nodes N_(i) in only a linear or smaller order relationship withincrease in the communication network size.

In the communication network control system 1 according to the presentexemplary embodiment 1, even when a node or a link in the communicationnetwork G is eliminated from such an original Ramanujan graph owing to afailure in the node or link, the same degree of efficiency androbustness as those of the original Ramanujan graph can be maintained.

In other words, by only eliminating a node N_(i) in which a failure hasoccurred (a breakdown node) from a Ramanujan graph with a size of n andchanging the connection of a link 2 having been connected to the nodeN_(i) in which a failure has occurred, it is possible to reconfigure aRamanujan graph with a size of (n−1) having approximately the samecharacteristics as those of the Ramanujan graph with a size of n.

Next, a configuration of the communication network system according tothe present exemplary embodiment 1 will be described in detail. In thecommunication network control system 1 according to the presentexemplary embodiment 1, when any node N_(i) in which a failure hasoccurred has been eliminated in the communication network G, each of theremaining nodes N_(i) performs reconnection in an autonomous anddistributed manner. Here, a description will be given of a case wherethe node degree k is 4.

FIG. 3 is a block diagram showing a schematic system configuration ofeach node in the communication network control system according to thepresent exemplary embodiment 1.

Each node N_(i) in the communication network control system 1 comprisesa communication function unit 11, a route information calculation unit12, a ranking information calculation unit 13, a determinationinformation calculation unit 14, a determination information comparisonunit 15 and a storage unit 16.

Here, for example, each node N_(i) is constituted by hardware whichmainly consists of a microcomputer composed of a CPU (Central ProcessingUnit) performing a control process, a computation process and the like,a ROM (Read Only Memory) storing a control program, a computationprogram and the like to be performed by the CPU, a RAM (Random AccessMemory) transitorily storing processing data or the like, and the like.The CPU, ROM and RAM are mutually connected via a data bus or the like.

The communication function unit 11 of each node N_(i) has a function asa communication interface for exchanging communication information withother nodes N_(i).

The route information calculation unit 12 of each node N_(i) is aspecific example of the route information calculation means, and itaccordingly calculates shortest route information M_(i) (i=0 to n−1) onthe shortest routes to respective ones of all nodes N_(i) located at a 2hop distance from the own node, on the basis of a route control protocolor the like, at a time when a route control has been completed in thecommunication network G.

Further, when a node N_(x) directly connected to the own node(hereafter, referred to as an adjacent node), among all nodes N_(i) inthe communication network G, breaks down, the route informationcalculation unit 12 of each node N_(i) calculates shortest routeinformation M_(i) in which the own node is set as the starting point.The route information calculation unit 12 stores the calculated routeinformation M_(i) into the storage unit 16.

The ranking information calculation unit 13 of each node N_(i) is aspecific example of the ranking information calculation means, and itaccordingly calculates ranking information Li (i=0 to n−1) in whichdestination candidate nodes N_(i) to which reconnection may be made areranked in descending order of their node-to-node distances in theshortest route information M_(i) calculated by the route informationcalculation unit 12.

The ranking information calculation unit 13 of each node N_(i)calculates the ranking information Li in which destination candidatenodes N_(j), N_(k) and N_(l) (j≠k≠l≠i≠x and j, k, l=0 to n−1) to whichreconnection (of the link 2 having been connected to the adjacent nodeN_(x) having broken down) may be made are ranked in descending order oftheir node-to-node distances in the shortest route information M_(i).The ranking information calculation unit 13 stores the calculatedranking information Li into the storage unit 16, along with the distanceinformation in the shortest route information M_(i) (FIG. 4).

When there is an adjacent node N_(x) having broken down, the rankinginformation calculation unit 13 of each node N_(i) exchanges thecalculated ranking information Li with all of the destination candidatenodes N_(j), N_(k) and N_(l), via the communication function unit 11.

The determination information calculation unit 14 of each node N_(i) isa specific example of the determination information calculation means,and it accordingly determines a reconnection destination of the own nodeN_(i) on the basis of the ranking information Li calculated by theranking information calculation unit 13. On the basis of the rankinginformation Li calculated by the ranking information calculation unit13, the determination information calculation unit 14 of each node N_(i)calculates a combination which makes largest the average of node-to-nodedistances in the shortest route information M_(i) over the entirecommunication network G after reconnection. Then, the determinationinformation calculation unit 14 determines the calculated combination ofconnections as a reconnection destination node of the own node N_(i) andstores the determination information Di (i=0 to n−1) into the storageunit 16.

The determination information calculation unit 14 of each node N_(i)sends the determination information Di having been determined, via thecommunication function unit 11, to the destination candidate nodesN_(j), N_(k) and N_(l) to which reconnection may be made.

The determination information comparison unit 15 of each node N_(i) is aspecific example of the determination information comparison means, andif it receives determination information Di from the reconnectiondestination node N_(k) via the communication function unit 11 and doesnot receive determination information Di from the other nodes N_(j) andN_(l) in the shortest route information M_(i) calculated by the routeinformation calculation unit 12, it fixes the determined reconnectiondestination node N_(k) as the reconnection destination. Thedetermination information comparison unit 15 of each node N_(i) storesthe fixed reconnection destination node N_(k) into the storage unit 16,as fixed information Fi (i=0 to n−1).

When the node N_(x) becomes unable to communicate owing to its breakdownor failure, the determination information comparison unit 15 of eachnode N_(i) re-sets a node N_(i) located at the next hop as thereconnection destination node N_(i) in the fixed information Fi, andthereby performs route control of the communication network G.

The storage unit 16 of each node N_(i) is a specific example of thestorage means, and it is constituted by, for example, theabove-described ROM or RAM.

FIG. 5 is a flow chart showing a flow of a communication network controlmethod according to the present exemplary embodiment 1.

At a time when route control has been completed in the communicationnetwork G, the route information calculation unit 12 of each node N_(i)calculates shortest route information M_(i) on the shortest routes torespective ones of all nodes N_(j), N_(k) and N_(l) located at a 2 hopdistance from the own node N_(i) on the basis of a route controlprotocol or the like. Further, when a node directly connected to the ownnode N_(i) among all nodes N_(i) in the communication network G, hasbroken down, the route information calculation unit 12 calculatesshortest route information M_(i) in which the own node N_(i) is set asthe starting point, and stores the calculated shortest route informationM_(i) into the storage unit 16 (step S101).

The ranking information calculation unit 13 of each node N_(i)calculates ranking information Li in which destination candidate nodesN_(j), N_(k) and N_(l) to which reconnection may be made are ranked indescending order of their distances in the shortest route informationM_(i) calculated by the route information calculation unit 12 (stepS102).

When there is an adjacent node N_(i) having broken down, the rankinginformation calculation unit 13 of each node N_(i) exchanges thecalculated ranking information Li with all of the destination candidatenodes N_(j), N_(k) and N_(l), via the communication function unit 11(step S103).

On the basis of the ranking information Li calculated by the rankinginformation calculation unit 13, the determination informationcalculation unit 14 of each node N_(i) calculates a combination whichmakes largest the average of node-to-node distances in the shortestroute information M_(i) over the entire communication network G afterreconnection. The determination information calculation unit 14determines the calculated combination of reconnections as a reconnectiondestination node of the own node N_(i), and stores determinationinformation Di according to the determination (i=0 to n−1) into thestorage unit 16 (step S104).

The determination information calculation unit 14 of each node N_(i)sends the determination information Di having been determined, via thecommunication function unit 11, to the destination candidate nodes towhich reconnection may be made (step S105).

If the determination information comparison unit 15 of each node N_(i)receives determination information Di from the reconnection destinationnode N_(k) via the communication function unit 11 and does not receivedetermination information Di from the other nodes N_(j) and N_(l) in theshortest route information M_(i) (NO at the step S106), it fixes thedetermined reconnection destination node N_(k) as the reconnectiondestination (step S107). The determination information comparison unit15 stores fixed information Fi indicating the fixed reconnectiondestination node N_(k) into the storage unit 16.

On the other hand, if, on the other hand, the determination informationcomparison unit 15 of each node N_(i) receives determination informationDi from the reconnection destination nodes N_(j) or N_(l) via thecommunication function unit 11 (YES at the step S106), the processreturns back to the above-described steps S103 or S104.

When its adjacent node N_(x) becomes unable to communicate owing to itsfailure or the like, the determination information comparison unit 15 ofeach node N_(i) re-sets a node N_(k) located at the next hop as thereconnection destination node based on the fixed information Fi, andthereby performs route control of the communication network G.

As has been described above, according to the communication networkcontrol system 1 according to the present exemplary embodiment 1, byonly eliminating a failure node N_(x) from a Ramanujan graph with a sizeof n and changing the connection of a link 2 having been connected tothe failure node N_(x), it is possible to reconfigure a Ramanujan graphwith a size of (n−1) having approximately the same characteristics asthose of the Ramanujan graph with a size of n. That is, even when afailure has occurred in a node or a link in the communication network G,it is possible to optimally reconstruct the communication network Gwhile maintaining the characteristics of the original graph.

Exemplary Embodiment 2

In contrast to that each node N_(i) performs reconnection in anautonomous and distributed manner in the exemplary embodiment 1 justdescribed above, an exemplary embodiment 2 described below ischaracterized by that a representative node N₀ performs reconnection ina centralized control manner. FIG. 6 is a diagram showing an example ofa communication network according to the exemplary embodiment 2 of thepresent invention. In the communication network G, a representative nodeN₀ to work as the central node is assigned in advance, to which aplurality of nodes N_(i) are connected via links 2.

FIG. 7 is a block diagram showing a schematic system configuration of arepresentative node in a communication network control system accordingto the present exemplary embodiment 2. The representative node N₀ in thecommunication network control system 1 comprises a communicationfunction unit 11, a route information calculation unit 12, a rankinginformation calculation unit 13, a determination information calculationunit 14 and a storage unit 16.

At a time when route control has been completed in the communicationnetwork G, the route information calculation unit 12 of therepresentative node N₀ calculates, with respect to each and every nodeN_(i) in the communication network G, shortest route information M_(i)(i=0 to n−1) on shortest routes to nodes located at a 2 hop distancefrom the own node, on the basis of a route control protocol or the like.The route information calculation unit 12 of the representative node N₀further calculates, with respect to each node set including a node N_(i)having broken down in the communication network G, shortest routeinformation M_(i) in which the node N_(i) included in the node set isset as the starting point. The route information calculation unit 12stores the calculated shortest route information M_(i) into the storageunit 16.

The ranking information calculation unit 13 of the representative nodeN₀ calculates ranking information Li (i=0 to n−1) based on the shortestroute information M_(i) with respect to destination candidate nodesN_(i) to which reconnection may be made. More specifically, when a nodeN_(x) in the communication network G has broken down, the rankinginformation calculation unit 13 of the representative node N₀calculates, with respect to destination candidate nodes N_(j), N_(k) andN_(l) to which reconnection of a link 2 having been connected to thefailure node N_(x) and to a node N_(i) adjacent to the failure nodeN_(x) may be made, the ranking information Li in which the candidatenodes are ranked in descending order of their distances in the shortestroute information M_(i) calculated by the route information calculationunit 12. Then, the ranking information calculation unit 13 stores thecalculated ranking information Li into the storage unit 16, along withthe distance information in the shortest route information M_(i) (FIG.8).

On the basis of the ranking information Li calculated by the rankinginformation calculation unit 13, the determination informationcalculation unit 14 of the representative node N₀ determines areconnection destination of each node N_(i). For example, on the basisof the ranking information Li calculated by the ranking informationcalculation unit 13, the determination information calculation unit 14calculates a combination of connections which makes largest the averageof distances in the shortest route information M_(i) over the entirecommunication network G after reconnection. Then, the determinationinformation calculation unit 14 determines the calculated combination ofconnections as a reconnection destination node of each node N_(i) andstores fixed information Fi (i=0 to n−1) representing the determinationresults into the storage unit 16.

When the node N_(x) has become unable to communicate owing to itsbreakdown or failure in the actual communication network G, thedetermination information calculation unit 14 of the representative nodeN₀ performs re-route control, on the basis of the fixed information Fi.

FIG. 9 is a flow chart showing a flow of a communication network controlmethod according to the present exemplary embodiment 2.

At a time when route control has been completed in the communicationnetwork G, the route information calculation unit 12 of therepresentative node N₀ calculates shortest route information M_(i) (i=0to n−1) with respect to each and every node N_(i) in the communicationnetwork G, on the basis of a route control protocol or the like.Further, when a node N_(x) in the communication network G has brokendown, the route information calculation unit 12 of the representativenode N₀ calculates, with respect to a set of adjacent nodes of the nodeN_(x) having broken down, shortest route information M_(i) in which eachof the nodes N_(i) included in the set is set as the starting point, andstores the calculated shortest route information M_(i) into the storageunit 16 (step S201).

The ranking information calculation unit 13 of the representative nodeN₀ calculates, with respect to destination candidate nodes N_(j), N_(k)and N_(l) to which reconnection of a link 2 having been connected to thenode N_(x) having broken down may be made, ranking information Li inwhich the destination candidate nodes are ranked in descending order oftheir distances in the shortest route information M_(i) calculated bythe route information calculation unit 12. Then, the ranking informationcalculation unit 13 stores the calculated ranking information Li intothe storage unit 16, along with the distance information in the shortestroute information M_(i) (step S202).

Here, for example, when the communication network G is large in size andis configured hierarchically, there should be a limit to a manageablerange of the representative node N₀, and accordingly, there may be acase where nodes N_(i) in a lower class cannot be managed. In that case,the present method may be configured such that a node in the lower classN_(i) sends ranking information Li to the representative node N₀ when afailure has occurred in the lower class node N_(i). The representativenode N₀ sends fixed information Fi, which will be described below, tothe lower class node N_(i). By that way, the communication network G canbe optimally reconstructed even when it is large in size.

On the basis of the ranking information Li calculated by the rankinginformation calculation unit 13, the determination informationcalculation unit 14 of the representative node N₀ calculates acombination of connections which makes largest the average ofnode-to-node distances in the shortest route information M_(i) over theentire communication network G after reconnection. Then, thedetermination information calculation unit 14 determines the calculatedcombination of connections as a reconnection destination node of eachnode N_(i), and stores fixed information Fi (i=0 to n−1) representingthe determination results into the storage unit 16 (step S203).

When a node N_(i) has become unable to communicate owing to itsbreakdown or failure in the actual communication network G, thedetermination information calculation unit 14 of the representative nodeN₀ performs re-route control, on the basis of the fixed information Fi(step S204).

As has been described above, in the communication network control system1 according to the present exemplary embodiment 2, by only eliminating anode N_(x) in which a breakdown or a failure has occurred, from aRamanujan graph with a size of n, and changing the connection of a link2 having been connected to the node N_(x) in which a breakdown or afailure has occurred, it is possible to reconfigure a Ramanujan graphwith a size of (n−1) having approximately the same characteristics asthose of the Ramanujan graph with a size of n. That is, even when afailure occurs in a node or a link in the communication network G, it ispossible to optimally reconstruct the communication network G whilemaintaining the characteristics of the original graph.

Here, the present invention is not limited to the above-describedexemplary embodiments, and can be appropriately modified within a rangenot departing from the spirit of the invention.

For example, while a Ramanujan graph is applied to the communicationnetwork G in each of the above-described exemplary embodiments 1 and 2,a graph applied to a communication network G is not limited to aRamanujan graph, and accordingly, a Cayley graph or a Petersen graph maybe applied, for example, and any graph belonging to expander graphs mayalso be applied.

While the route information calculation unit 12 calculates shortestroute information M_(i) in each of the above-described exemplaryembodiments 1 and 2, route information to be calculated is not limitedto such shortest route information, and accordingly, the routeinformation calculation unit 12 may calculate, for example, routeinformation based on a band or a delay, or both of them. In such a case,the ranking information calculation unit 13 calculates rankinginformation Li in which ranking is made in descending order of distancesin the route information M_(i) calculated by the route informationcalculation unit 12.

While, in each of the above-described exemplary embodiments 1 and 2, thedetermination information calculation unit 14 calculates a combinationwhich makes largest the average of node-to-node distances in theshortest route information M_(i) over the entire communication network Gafter reconnection, on the basis of the ranking information Licalculated by the ranking information calculation unit 13, a combinationto be calculated is not limited to that one, and accordingly, thedetermination information calculation unit 14 may calculate acombination which makes the average of node-to-node distances in theshortest route information M_(i) equal to or larger than a predeterminedthreshold value.

Further, the present invention can be realized by causing a CPU toexecute a computer program for performing the process shown in FIG. 5 orthat in FIG. 9.

The program may be stored using various types of non-transitory computerreadable media and may then be supplied to a computer. Thenon-transitory computer readable media include various types of tangiblestorage media. Examples of the non-transitory computer readable mediainclude magnetic recording media (for example, a flexible disc, amagnetic tape and a hard disk drive), magneto-optic recording media (forexample, a magneto-optic disk), a CD-ROM (Read Only Memory), a CD-R, aCD-R/W, semiconductor memories (for example, a mask ROM, a PROM(Programmable ROM) and an EPROM (Erasable PROM), a flash ROM and a RAM(Random Access Memory).

Besides, the program may also be supplied by various types of transitorycomputer readable media to a computer. Examples of the transitorycomputer readable media include electrical signals, optical signals andradio waves. Such transitory computer readable media may supply theprogram to a computer via a wired communication line such as an electricwire or an optical fiber, or a wireless communication line.

Part or the whole of the above-described exemplary embodiments can bealso described as, but is not limited to, the following supplementarynotes.

(Supplementary Note 1)

A communication network control system for, in the communication networkwith a plurality of nodes mutually connected via a plurality of links,eliminating any of the nodes having broken down or having a failure andperforming reconnection within the communication network,

wherein at least one of the plurality of nodes comprises:

a route information calculation means for calculating route informationin the communication network when a failure occurs in an adjacent node;

a ranking information calculation means for calculating, with respect todestination candidate nodes to which the reconnection may be made,ranking information in which the destination candidate nodes are rankedin order of their distances in the route information calculated by theroute information calculation means; and

a determination information calculation means for calculating, on thebasis of the ranking information calculated by the ranking informationcalculation means, a combination which causes distances in the routeinformation to be large over the entire communication network after thereconnection, and determining the calculated combination as areconnection destination node of the own node.

(Supplementary Note 2)

The communication network control system according to supplementary note1, wherein

the determination information calculation means calculates, on the basisof the ranking information calculated by the ranking informationcalculation means, a combination which makes the average of node-to-nodedistances in the route information over the entire communication networkafter the reconnection equal to or larger than a predetermined thresholdvalue.

(Supplementary Note 3)

The communication network control system according to supplementary note1 or 2, wherein

the determination information calculation means calculates, on the basisof the ranking information calculated by the ranking informationcalculation means, a combination which makes largest the average ofnode-to-node distances in the route information over the entirecommunication network after the reconnection.

(Supplementary Note 4)

The communication network control system according to any one ofsupplementary notes 1 to 3, wherein

each and every one of the plurality of nodes comprises

the route information calculation means, the ranking informationcalculation means and the determination information calculation means,wherein

the route information calculation means calculates shortest routeinformation in which the own node is set as the starting point.

(Supplementary Note 5)

The communication network control system according to supplementary note4, wherein,

when there is an adjacent node in which a failure has occurred, theranking information calculation means exchanges the calculated rankinginformation with all destination nodes to which reconnection may bemade.

(Supplementary Note 6)

The communication network control system according to any one ofsupplementary notes 1 to 5, the communication network control systemfurther comprising

a determination information comparison means for fixing the reconnectiondestination node determined by the determination information calculationmeans of the own node as the reconnection destination, if havingacquired determination information from the reconnection destinationnode and not having acquired determination information from the othernodes in the route information.

(Supplementary Note 7)

The communication network control system according to supplementary note6, wherein,

when an adjacent node has a failure and thereby becomes unable tocommunicate, the determination information comparison means re-sets anode of the next hop as the reconnection destination, and therebyperforms route control.

(Supplementary Note 8)

The communication network control system according to any one ofsupplementary notes 1 to 7, the communication network control systemfurther comprising

a storage means for storing at least either of route informationcalculated by the route information calculation means and rankinginformation calculated by the ranking information calculation means.

(Supplementary Note 9)

The communication network control system according to any one ofsupplementary notes 1 to 8, wherein

the nodes perform the reconnection in an autonomous and distributedmanner.

(Supplementary Note 10)

The communication network control system according to any one ofsupplementary notes 1 to 3, wherein

a representative node assigned in advance from among the plurality ofnodes comprises

the route information calculation means, the ranking informationcalculation means and the determination information calculation means,wherein

the route information calculation means calculates, with respect to aset of nodes whose adjacent node has broken down, shortest routeinformation in which each node in the node set is set as the startingpoint.

(Supplementary Note 11)

The communication network control system according to supplementary note10, wherein,

when the node has a failure and thereby becomes unable to communicate,the determination information calculation means performs re-routecontrol on the basis of the determination information indicating areconnection destination node.

(Supplementary Note 12)

The communication network control system according to any one ofsupplementary notes 1 to 11, wherein

a Ramanujan graph is applied to the communication network.

(Supplementary Note 13)

A control method of a communication network control system for, in thecommunication network with a plurality of nodes mutually connected via aplurality of links, eliminating any of the nodes having broken down orhaving a failure and performing reconnection within the communicationnetwork,

the control method comprising:

a step of calculating route information in the communication networkwhen a failure occurs in an adjacent node;

a step of calculating, with respect to destination candidate nodes towhich the reconnection may be made, ranking information in which thedestination candidate nodes are ranked in order of their distances inthe calculated route information; and

a step of calculating, on the basis of the calculated rankinginformation, a combination which causes distances in the routeinformation to be large over the entire communication network after thereconnection, and determining the calculated combination as areconnection destination node of the own node.

(Supplementary Note 14)

A control program of a communication network control system for, in thecommunication network with a plurality of nodes mutually connected via aplurality of links, eliminating any of the nodes having broken down orhaving a failure and performing reconnection within the communicationnetwork,

the control program causing a computer to execute:

a process of calculating route information in the communication networkwhen a failure occurs in an adjacent node;

a process of calculating, with respect to destination candidate nodes towhich the reconnection may be made, ranking information in which thedestination candidate nodes are ranked in order of their distances inthe calculated route information; and

a process of calculating, on the basis of the calculated rankinginformation, a combination which causes distances in the routeinformation to be large over the entire communication network after thereconnection, and determining the calculated combination as areconnection destination node of the own node.

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2013-001308, filed on Jan. 8, 2013, thedisclosure of which is incorporated herein in its entirety by reference.

REFERENCE SIGNS LIST

-   -   1 communication network control system    -   2 link    -   3 route information calculation means    -   4 ranking information calculation means    -   5 determination information calculation means    -   11 communication function unit    -   12 route information calculation unit    -   13 ranking information calculation unit    -   14 determination information calculation unit    -   15 determination information comparison unit    -   16 storage unit    -   G communication network    -   N_(i) node    -   N₀ representative node    -   N_(x) node having broken down or having failure    -   N_(j), N_(k), N_(l) adjacent node of node having broken down or        having failure

The invention claimed is:
 1. A communication network control system for,in a communication network with a plurality of nodes mutually connectedvia a plurality of links, eliminating any of the nodes having brokendown or having a failure and performing reconnection within thecommunication network, wherein at least one of the plurality of nodescomprises: a route information calculation unit for calculating routeinformation in the communication network when a failure occurs in anadjacent node; a ranking information calculation unit for calculating,with respect to a set of destination candidate nodes to which thereconnection may be made, ranking information in which the destinationcandidate nodes are ranked in order of their node-to-node distances inthe route information calculated by the route information calculationunit; and a determination information calculation unit for calculating,on the basis of the ranking information calculated by the rankinginformation calculation unit, a combination which causes node-to-nodedistances in the route information to be large over the entirecommunication network after the reconnection, and determining thecalculated combination as a reconnection destination node of the ownnode, and wherein a Ramanujan graph is applied to the communicationnetwork.
 2. The communication network control system according to claim1, wherein the determination information calculation unit calculates, onthe basis of the ranking information calculated by the rankinginformation calculation unit, a combination which makes the average ofnode-to-node distances in the route information over the entirecommunication network after the reconnection equal to or larger than apredetermined threshold value.
 3. The communication network controlsystem according to claim 1, wherein the determination informationcalculation unit calculates, on the basis of the ranking informationcalculated by the ranking information calculation unit, a combinationwhich makes largest the average of node-to-node distances in the routeinformation over the entire communication network after thereconnection.
 4. The communication network control system according toclaim 1, wherein each and every one of the plurality of nodes comprisesthe route information calculation unit, the ranking informationcalculation unit and the determination information calculation unit,wherein the route information calculation unit calculates shortest routeinformation in which the own node is set as the starting point.
 5. Thecommunication network control system according to claim 4, wherein, whenthere is an adjacent node in which a breakdown or a failure hasoccurred, the ranking information calculation unit exchanges thecalculated ranking information with all destination nodes to whichreconnection may be made.
 6. The communication network control systemaccording to claim 1, the communication network control system furthercomprising a determination information comparison unit for fixing thereconnection destination node determined by the determinationinformation calculation unit of the own node as the reconnectiondestination, if having acquired determination information from thereconnection destination node and not having acquired determinationinformation from the other nodes in the route information.
 7. Thecommunication network control system according to claim 6, wherein, whenan adjacent node has a breakdown or a failure and thereby becomes unableto communicate, the determination information comparison unit re-sets anode of the next hop as the reconnection destination, and therebyperforms route control.
 8. The communication network control systemaccording to claim 1, the communication network control system furthercomprising a storage unit for storing at least either of routeinformation calculated by the route information calculation unit andranking information calculated by the ranking information calculationunit.
 9. The communication network control system according to claim 1,wherein the nodes perform the reconnection in an autonomous anddistributed manner.
 10. The communication network control systemaccording to claim 1, wherein a representative node assigned in advancefrom among the plurality of nodes comprises the route informationcalculation unit, the ranking information calculation unit and thedetermination information calculation unit, wherein the routeinformation calculation unit calculates, with respect to a set of nodeswhose adjacent node has broken down, shortest route information in whicheach node in the node set is set as the starting point.
 11. Thecommunication network control system according to claim 10, wherein,when the node has a breakdown or a failure and thereby becomes unable tocommunicate, the determination information calculation unit performsre-route control on the basis of the determination informationindicating a reconnection destination node.
 12. A control method of acommunication network control system for, in a communication networkwith a plurality of nodes mutually connected via a plurality of links,eliminating any of the nodes having broken down or having a failure andperforming reconnection within the communication network, the controlmethod comprising: calculating route information in the communicationnetwork when a failure occurs in an adjacent node; calculating, withrespect to destination candidate nodes to which the reconnection may bemade, ranking information in which the destination candidate nodes areranked in order of their distances in the calculated route information;and calculating, on the basis of the calculated ranking information, acombination which causes distances in the route information to be largeover the entire communication network after the reconnection, anddetermining the calculated combination as a reconnection destinationnode of the own node, wherein a Ramanujan graph is applied to thecommunication network.
 13. A non-transitory computer readable medium ofcontrol program of a communication network control system for, in acommunication network with a plurality of nodes mutually connected via aplurality of links, eliminating any of the nodes having broken down orhaving a failure and performing reconnection within the communicationnetwork, the control program causing a computer to execute: a process ofcalculating route information in the communication network when afailure occurs in an adjacent node; a process of calculating, withrespect to destination candidate nodes to which the reconnection may bemade, ranking information in which the destination candidate nodes areranked in order of their node-to-node distances in the calculated routeinformation; and a process of calculating, on the basis of thecalculated ranking information, a combination which causes node-to-nodedistances in the route information to be large over the entirecommunication network after the reconnection, and determining thecalculated combination as a reconnection destination node of the ownnode, wherein a Ramanujan graph is applied to the communication network.14. A communication network control system for, in a communicationnetwork with a plurality of nodes mutually connected via a plurality oflinks, eliminating any of the nodes having broken down or having afailure and performing reconnection within the communication network,wherein at least one of the plurality of nodes comprises: a routeinformation calculation means for calculating route information in thecommunication network when a failure occurs in an adjacent node; aranking information calculation means for calculating, with respect todestination candidate nodes to which the reconnection may be made,ranking information in which the destination candidate nodes are rankedin order of their distances in the route information calculated by theroute information calculation means; and a determination informationcalculation means for calculating, on the basis of the rankinginformation calculated by the ranking information calculation means, acombination which causes distances in the route information to be largeover the entire communication network after the reconnection, anddetermining the calculated combination as a reconnection destinationnode of the own node, and wherein a Ramanujan graph is applied to thecommunication network.